1. Field of the Invention
The present invention relates to a laser drawing apparatus which is adapted for example to form a predetermined pattern of circuit on a circuit board.
2. Description of the Related Art
In a known method of forming a circuit pattern on a circuit board (i.e., substrate), a photopolymer or the like is uniformly applied to the substrate coated with a thin film layer of electrically conductive metal, such as copper. Thereafter, the substrate is illuminated with ultraviolet light, for example, while masking the substrate with an exposing and printing photomask (photomask film) having a predetermined shape, so that a circuit pattern corresponding to the photomask is formed on the substrate. The exposed photopolymer on the substrate is dissolved by a solvent and is subjected to a predetermined treatment by chemicals in liquid state so that the exposed conductive metal is corroded. No corrosion occurs at the portion of the substrate on which the non-exposed photopolymer layer remains. Hence, the same circuit pattern as the photomask pattern is formed on the substrate.
However, in the known manufacturing method as mentioned above, it requires a long time and a number of processes to examine the indispensable photomask. Furthermore, it is necessary not only to create the environment for the photomask in which the temperature and humidity are kept constant to thereby prevent the photomask from being thermally contracted or expanded, but also to protect the photomask from dust or being damaged, thus resulting in a heavy burden on the maintenance of the photomask.
It is also known to directly draw (expose) the circuit pattern on the substrate, using a scanning laser beam with which the substrate is scanned with the help of a polygonal mirror or the like, without using an exposing and printing photomask. In this method, the above-mentioned drawbacks in the manufacturing method in which the photomask is employed can be eliminated, but there is another problem when the laser beam is reflected by a polygonal mirror or the like and is converged onto the substrate through a lens.
Namely, when the laser beam is scanned through the lens at the meridian thereof, there is no problem, but when the laser beam passes through the portion of the lens other than the meridian thereof, the laser beam emitted from the lens is deviated or bent, thus resulting in a distorted image. The deviation or distortion increases as the distance of the incident point from the meridian increases.